Because of excellent heat resistance and electrical insulation, polyimide resins are widely used as resin varnish for electronic parts and base material for flexible printed circuit boards. However, the polyimide resins suffer from several problems including the lack of flexibility due to rigidity, awkward use because of a high glass transition temperature, and minimized solubility in organic solvents. Then a variety of silicone-modified polyimide resins have been proposed, as described, for example, in JP-A 10-195278 and JP-A 8-34851. These silicone-modified polyimide resins are improved in substrate bond and electrical properties while overcoming the above problems.
The synthesis of prior art silicone-modified polyimide resins is carried out, for example, by reacting an acid dianhydride with a diamine compound to form a polyamic acid, followed by ring-closing polyimide-forming reaction at high temperatures above 150° C. That is, the synthesis uses rigorous conditions and takes a long time. It would be desirable to have a resin material which has at least equal function to the prior art silicone-modified polyimide resins, can be synthesized more easily, and has the property of thermosetting. In this regard, the inventor discovered that an imide silicone resin is readily synthesized through hydrosilylation which is a relatively moderate reaction (see Japanese Patent Application No. 2002-259317). A thermoset resin coating resulting from this imide silicone resin has excellent resistance to solvent, humidity and the like, but requires a longer time to cure when the curing temperature is of the order of 100° C. A curing temperature in excess of 200° C. is necessary to complete brief curing. This limits the range of material and application with which the resin can be used.
A composition comprising a bisnadimide-polysiloxane alternating copolymer and an epoxy resin has been proposed as finding use in electronic materials, especially semiconductor-encapsulation resins (see, for example, JP-A 2003-20337). In this composition, however, the bisnadimide-polysiloxane alternating copolymer is used merely as an additive for stress reduction and heat resistance improvement rather than reacting with the epoxy resin. This indicates a likelihood that the passage of time induces phase separation between the bisnadimide-polysiloxane alternating copolymer and the epoxy resin, leading to a loss of reliability.